1. Field of the Invention
This invention generally relates to the formation of electronic resistors and, more particularly, to the adjustment of resistance accuracy of resistors formed for use with integrated circuits.
2. Description of the Prior Art
Resistors are a very basic element of many electrical and electronic circuits. All materials exhibit a characteristic resistance to the flow of electricity therethrough, which is referred to as the bulk resistance of the material. Exploiting this property of some materials having a moderate bulk resistance value, electrical resistors usually consist of a volume of material having a predetermined cross-sectional area and a predetermined length with respect to the locations on that volume of material where electrodes are applied. Length can be generally taken to be the distance between the terminals and the cross-sectional area can generally be measured in a plane perpendicular to the length. The actual value of elctrical resistance for a resistor formed of a given material will vary proportionally with length and inversely with cross-sectional area.
In the manufacture of resistors, the precision of the dimensions of the resistive element will affect the accuracy of the resistance value as compared with the intended design value. Therefore resistors are commonly available having different resistance tolerances, 10%, 5% and 1% being typical. Higher accuracy is typically achieved by a process known as trimming, which, as the name implies, generally involves cutting away of a portion of the cross-section of the resistive element to increase the resistance to an exactly desired value.
With the development of complex integrated circuits, it has become common to fabricate one or more resistors, including networks and arrays of transistors on a single substrate, often in combination with active integrated circuit elements such as transistors. A digital-to-analog converter circuit is an example of a circuit which may include a resistance array as well as digital logic elements and analog amplifier and feedback circuits on a single chip. However, many digital logic circuits will also typically contain at least a few resistors, often for the purpose of adjusting switching threshold voltages or output levels (e.g. pull-up or pull-down resistors.
For such miniaturized applications, resistive elements are often formed of thin layers of metal, doped silicon or metal oxides of predetermined widths on the substrate or within the structure of the semiconductor device. In such constructions, it is common to refer to the sheet resistance of the material. Trimming is often even more literally done by trimming away a portion of the width of the material layer. This has commonly been done by etching but, as devices have become smaller, lasers have been employed to vaporize or ablate small areas of the deposited resistive material. However, in extremely small devices and in highly sophisticated integrated circuits, such intense local applications of heat have often not yielded sufficiently accurate adjustment of the resistance of a resistor formed therein. Such intense heat has also led to other problems, occasionally destructive of the integrated circuit device itself.
Specifically, when the resistive element is cut by the laser, some of the material may melt without being vaporized and bridge the kerf in the resistor which is cut by the laser. This causes a smaller than intended change in the resistor value. The kerf width may also vary somewhat unpredictably because of underlying device topology which alters the degree of heating caused by a predetermined amount of laser power applied. Further, surface contamination can interfere with the actual application of heat to the material with a laser, either by reflecting energy or causing it to be absorbed in greater than the expected degree. Similarly, depending on the wavelength of the laser radiation, there may be constructive or destructive interference in the vicinity of the resistive element due to variations in underlying quartz layer thicknesses. It must be recognized that irregularities in the laser kerf can cause current crowding which will result in "hot spots" in the resulting resistor, when operated. This "hot spots" effect will result in a reduction of the reliability of the resistor (since it will then resemble a fuse) and possibly aggravate temperature dependent resistance change.
Also, such localized heating may cause delamination of the resistor or other integrated circuit layers from underlying layers or micro cracking of the substrate or layers of the integrated circuit. Such structural defects in the integrated circuit may cause circuit discontinuities, localized power dissipation problems leading to premature failure of the device and other effects adversely impacting the functionality, reliability and operating margins of the integrated circuit.